It is known in the art to provide a liquid level sensor comprising a non-magnetic guide tube vertically arranged in the liquid whose level is to be sensed, with a plurality of reed switches disposed spaced apart in the tube to be actuated by a magnet carried by a float guided on the tube and rising and falling with the liquid level. The reed switches are connected in electric circuits to signal the liquid level, and may serve to cause energization of pumps, alarms, indicators and the like. The following United States patents are representative of such teaching:
______________________________________ 3,200,645 3,646,293 3,788,340 3,678,750 3,685,357 ______________________________________
A problem common to each of these is that while the reed switch is closed as the float magnet passes, as for example during a rise of the float, it then opens as soon as the float magnet passes above the switch. Consequently, the only time a circuit is completed through the reed switch is when the float magnet field is sweeping it, and in order to be useful the reed switch must therefor be electrically connected to a system which is undisturbed by the switch opening as the float continues to rise. Such a system may utilize a latching relay as in U.S. Pat. No. 3,685,357 which will maintain a circuit energized even though the reed switch opens. Redundancy problems are raised by such arrangements to insure that when the reed switch opens it is not because the float has reversed direction but is actually still rising. In addition, should a float be rising and before reaching a reed switch there be an electric power failure during the time the float passes the switch, the circuit will not indicate the increased liquid level when the power returns.
To avoid redundancy problems, power failure problems, latching relays and the like, efforts have been directed to devising means for maintaining the reed switch closed when the float magnet reaches it and despite the liquid level continuing to rise. One commercially available solution, and another shown in U.S. Pat. No. 3,826,139, involves the use of a guide tube having a plurality of floats, one for each reed switch, and stops on the guide tube which limit the rise of each float so that its ascent is arrested when its magnet has closed the reed switch. The float then remains in this position as the liquid continues to rise thus holding the reed switch closed. The switch is opened when the liquid level falls sufficiently to allow the float to drop away from the stop and carry its magnet sufficiently below the reed switch so it can open. In addition to the necessity of using a number of floats, which increases the cost of the system, the only way the sensing levels can be changed is by physically shifting the float stops on the guide tube and this entails gaining physical access to the outside of the guide tube which is sometimes difficult or inconvenient.
Another solution is proposed by U.S. Pat. No. 3,437,771 where a two-part float is shown, one part carrying a magnet is intended to lock onto a bias magnet at the switch and open the switch and remain at the switch until the other float part drops sufficiently to carry the inner float down away from the switch and allow the switch to re-close. This teaching also uses a stop to limit rise of the float, and would require multiple floats and stops if more than two switches (levels) were involved. In addition, the switch is normally closed by the bias magnet and is opened by the float magnet, and the teaching would appear to be limited to this mode of operation.